Colonization of the human tooth surface is initiated by a limited number of gram-positive species, primarily different viridans group streptococci. These bacteria attach to host salivary components that coat the mineral surface and, through growth and interactions between species, form a relatively simple biofilm community (i.e. early dental plaque). Members of this community can activate host cells, and the biofilm itself creates a habitat for additional species, some of which are closely associated with the initiation or progression of dental caries and periodontal disease. Bacterial interactions that contribute to early biofilm development include those detected by in vitro coaggregations between different viridans group streptococci and actinomyces. These interactions generally depend on lectin-like cell surface adhesins present on actinomyces and complementary receptor polysaccharides (RPS) present on streptococci. Extensive structural studies have resulted in the identification of six different types of RPS, each composed of a distinct hexa- or heptasaccharide repeating unit. Each RPS repeating unit contains a host-like motif that functions as a recognition site for adhesin binding. Other structural features account for the reactions of these polysaccharides as antigens. The occurrence and distribution of these polysaccharides on different viridans group streptococci may influence oral biofilm development. Large gene clusters that direct the synthesis of different RPS structural types have also been identified. Each cluster contains four common regulatory genes. These are followed by additional genes for the glycosyl transferases that synthesize the lipid-linked oligosaccharides that are the building blocks of each RPS. Other genes encode a flipase that transports the oligosaccharide moiety from the inner to the outer surface of the cytoplasmic membrane and a polymerase that links these moieties end-to-end forming the linear polysaccharide chain. Genes that control the synthesis of essential nucleotide-linked sugars have also been identified. One of these encodes galactofuranose mutase, the enzyme that catalyzes the synthesis of UDP-Galactofuranose, an essential RPS precursor. This gene has been found within the RPS gene clusters of all streptococcal species examined to date and is thus, a useful marker for the identification of these regions. In contrast, the gene for an essential bifunctional galactose epimerase occurs outside RPS gene clusters. The location of other genes for nucleotide sugar biosynthesis varies between species. For example, the genes for dTDP-L-Rhamnose biosynthesis are associated with the RPS gene clusters of Streptococcus oralis but not with the RPS gene clusters of Streptococcus gordonii. Differences in the location and arrangement of these genes are of interest as they may provide insight into the evolution of different streptococci as members of oral biofilm communities. Further insights into the molecular basis of RPS structure and function have been gained from the recent production and characterization of genetically modified polysaccharides. These polysaccharides were engineered by replacing genes in one RPS gene cluster with different genes from another RPS gene cluster and characterized by high resolution nuclear magnetic resonance. The combined results of these molecular and structural studies have resulted in the unambiguous identification of genes for the glycosyl transferases that synthesize the recognition motifs and antigenic epitopes for different RPS structural types. The results also illustrate a new approach for studying the acceptor specificities of glycosyl transferases. The ability to genetically engineer bacterial surface carbohydrates has a wide range of potential applications. The limits of this emerging technology will ultimately be determined by the donor and acceptor specificities of the glycosyl transferases and polymerases encoded by available genes. The structural complexities of the surface polysaccharides present on oral viridans group streptococci suggest that these bacteria represent a rich source of such genes. Moreover, the likelihood that these genes can be used to engineer novel carbohydrate structures is increased by the finding that the acceptor specificity of certain glycosyl transferases involved in RPS biosynthesis appears to be less strict than initially anticipated. Further studies of streptococcal RPS gene clusters are underway to identify genetic markers for oral biofilm development and also to explore the limits of carbohydrate engineering in this experimental system.